It is known that the fuel injection, in which the fuel is injected into a cylinder of an internal combustion engine, is carried out in a divided manner a plurality of times. In this case, unburned fuel can be discharged to an exhaust passage by carrying out the post injection after the main injection. The post injection is performed, for example, when the position of a piston is relatively low in the vicinity of the end of the expansion stroke. On this account, the fuel, which is injected by the post injection, hardly contributes to the torque of the internal combustion engine. Then, the heat can be generated by reacting the unburned fuel discharged to the exhaust passage after the post injection by using a catalyst to regenerate a filter by utilizing the heat. Further, the unburned fuel can be supplied as a reducing agent to the catalyst to perform, for example, the reduction of NOx. However, the post injection is carried out when the position of the piston is relatively low, and hence it is feared that the fuel, which is injected by the post injection, may arrive at a wall surface of the cylinder to dilute oil.
For example, when the load exerted on the internal combustion engine is low, then the temperature in the cylinder, which is provided when the fuel injected by the main injection is combusted, is low, and hence the fuel is hardly evaporated. Further, when the load exerted on the internal combustion engine is low, the pressure of the fuel is set to be low. Therefore, in such a case, the particle diameter (particle size) of the injected fuel is increased, and the fuel is hardly evaporated. Further, when the load exerted on the internal combustion engine is low, the temperature of the gas discharged from the cylinder is low. Therefore, the amount of the fuel, which is provided during the post injection and which is required to raise the temperature of the catalyst, is increased as well. On this account, the fuel is hardly evaporated.
On the other hand, when the load exerted on the internal combustion engine is high, the injection pressure of the fuel is set to be high in order to reduce the smoke. The post injection is performed, for example at a timing near to the end of the expansion stroke. Therefore, the post injection is performed after the pressure in the cylinder is lowered. In such a situation, the pressure of the fuel is relatively raised as compared with the pressure in the cylinder, and hence the penetration force of the fuel is raised. On this account, the time, which is required until the fuel arrives at the wall surface of the cylinder, is shortened. Therefore, the fuel, which is injected by the post injection, easily arrives at the wall surface of the cylinder before being evaporated.
In this context, a technique is known, in which the dilution of the oil is suppressed by selecting a smaller injection amount from the injection amount at which the fuel injected by the post injection does not arrive at the wall surface of the cylinder in a state of liquid and the injection amount which is requested from the temperature of the catalyst (see, for example, Patent Literature 1).